Aircraft spoilers are typically actuated by linear actuators. When an aircraft is cruising, the airflow over the wing exerts a suction force on the spoilers, which (if not resisted) would cause the spoilers to lift away from the upper surface of the wing. This would disrupt the aerodynamic profile of the wing, and would consequently reduce the fuel efficiency of the aircraft. The spoiler actuators are therefore arranged to resist this suction force and hold the spoilers in a fully retracted configuration whilst the aircraft is cruising.
Over time, a spoiler actuator may become less effective at holding the spoiler down against the aerodynamic suction force (e.g. because they are in need of maintenance or replacement). This may not affect the actuators ability to deploy and position the spoiler when deployed, so it may not be noticed by flight or ground crew. However; it can negatively impact the fuel efficiency of the aircraft, as described above, and so it is desirable to detect when a spoiler actuator has become less effective at holding the spoiler in the fully retracted position.
Known techniques for testing the operation of linear actuators cannot be performed on an actuator which is installed on an aircraft. Instead, the actuator must be removed from the aircraft for testing. This makes such techniques unsuitable for routine testing, such as would be necessary to detect relatively small decreases in actuator effectiveness.